The present invention relates to an MEMS element, a GLV device, and a laser display. More particularly, the invention relates to an MEMS element having such a structure as not to be susceptible to damage at the time of removing a sacrificing layer, particularly an MEMS element having a high light reflectance optimum as a light modulation element, a GLV device comprising the MEMS elements, and a laser display comprising the GLV device.
Attendant on the progress of microtechnology, attention has been paid to the so-called micromachine (MEMS: Micro Electro-Mechanical System) element (hereinafter referred to as MEMS element).
The MEMS element is formed as a microstructure on a substrate, such as a silicon substrate, a glass substrate, and the like, in which a driver for outputting a mechanical driving force and a semiconductor integrated circuit or the like for controlling the driving of the driver are coupled electrically and mechanically. A basic characteristic of the MEMS element lies in that the driver constituted as a mechanical structure is incorporated in a part of the element, and the driving output of the driver is generally performed electrically by application of a coulomb force between electrodes.
As an example of the MEMS element, a light modulation element used in a GLV (Grating Light Valve) device developed as a light intensity conversion element for laser display, namely, as a light modulator, by the SLM (Silicon Light Machine) Company is mentioned, and its structure will be described.
First, referring to FIG. 5, the structure of the GLV device comprised of the MEMS elements will be described. FIG. 5 is a perspective view showing the constitution of the GLV device.
As shown in FIG. 5, the GLV device 10 is a device in which a plurality of MEMS elements 12 are disposed on a common substrate densely and in parallel to each other.
Each of the MEMS elements 12 constituting the GLV device is an MEMS element called MOEMS (Micro Optical Electro-Mechanical Systems) comprising an electrostatic driving type membrane 16 having a light reflective surface 14 on the upper side thereof and has the function of modulating the intensity of the reflected light by regulating the distance between the light reflective surface 14 and the substrate 18 through mechanical movements of the membrane 16 by an electrostatic attracting force or an electrostatic repulsion force.
Next, referring to FIG. 6, the constitution of the MOEMS 12 will be described. FIG. 6 is a perspective view showing the constitution of the MOEMS.
As shown in FIG. 6, the MOEMS 12 comprises an insulating substrate 18, such as a glass substrate, a substrate-side electrode 20 comprised of a thin Cr film or the like and formed on the insulating substrate 18, and the electrostatic driving type membrane 16 crossing and striding the substrate-side electrode 20 in a bridge form.
The electrostatic driving type membrane 16 and the substrate-side electrode 20 are electrically isolated from each other by a void portion 22 therebetween.
The electrostatic driving type membrane 16 comprises a bridge member 24 composed of an SiN film provided as an electrode support member and based on the substrate 18 bridgingly astride the substrate-side electrode 20 and a combined light reflective film and membrane-side electrode 14 composed of an Al film with a thickness of about 100 nm and provided on the bridge member 24 opposed to and in parallel to the substrate-side electrode 20.
The bridge member 24 is opposed to and spaced by a predetermined distance from the substrate-side electrode 20 so as to secure the void portion 22 therebetween, and it is provided for supporting the combined light reflective film and membrane-side electrode 14 in parallel to the substrate-side electrode 20.
In the GLV device 10, the insulating substrate 18 and the substrate-side electrode 20 thereon are respectively a common substrate and a common electrode for the MOEMS 12, as shown in FIG. 5.
The electrostatic driving type membrane 16 constituted of the bridge member 24 and the combined light reflective film and membrane-side electrode 14 thereon is a portion called a ribbon. The bridge member 24 may in some cases be of the cantilever type in which only one end of a beam portion is supported by one column portion, in place of the bridge form shown in FIG. 6 in which both ends of a beam portion extending in parallel to the substrate-side electrode 20 are supported by two column portions, respectively.
The aluminum film (Al film) used as the combined light reflective film and membrane-side electrode 14 is a metallic film preferable as an optical component material on the grounds that (1) it is a metallic film which can be formed comparatively easily, (2) it has a small wavelength dispersion of light reflectance in the visible ray region, (3) a spontaneously oxidized alumina film formed on the surface of the Al film functions as a protective film for protecting the reflective surface, and the like.
On the other hand, the SiN film (silicon nitride film) constituting the bridge member 24 is selected on the ground that its physical properties, such as strength and elastic constant, are suitable for mechanical driving of the bridge member 24.
When a minute voltage is impressed between the substrate-side electrode 20 and the combined light reflective film and membrane-side electrode 14 opposed to the substrate-side electrode 20, the electrostatic driving type membrane 16 approaches the substrate-side electrode 20 due to an electrostatic phenomenon, and when the impressing of the voltage is stopped, the electrostatic driving type membrane 16 is spaced away from the substrate-side electrode 20 into its original state.
The MOEMS 12 constituting the GLV device 10 each functions as a light modulation element for modulating the intensity of reflected light by changing the inclination of the combined light reflective film and membrane-side electrode 14 through the approaching and spacing of the electrostatic driving type membrane 16 relative to the substrate-side electrode 20.
The dynamic characteristics of the membrane 16 driven by utilizing the electrostatic attracting force and the electrostatic repulsion force are substantially determined by the physical properties of the SiN film formed by the CVD method or the like, and the Al film plays a main role as a reflection mirror.
The void structure necessary for the membrane 16 to function as the driver is formed by a process in which a sacrificing layer to be finally removed is formed between the SiN film constituting the bridge member 24 and the substrate-side electrode 20 on the lower side, and, after the formation of the membrane 16, only the sacrificing layer is selectively removed.
Next, referring to FIG. 7, a method of fabricating the MOEMS 12 will be described. FIGS. 7A to 7E are sectional views taken along line I—I of FIG. 6 according to the steps of fabrication of the MOEMS 12.
As shown in FIG. 7A, a metallic film, such as a W (tungsten) film, is formed on the substrate 18, and it is patterned to form the substrate-side electrode 20.
Next, as shown in FIG. 7B, an amorphous silicon film or a polysilicon film is formed on the entire surface of the substrate 18, and it is patterned to form the sacrificing layer 26 on the substrate-side electrode 20.
The sacrificing layer 26 functions as a support layer for forming the bridge member 20 next and is finally removed, as will be described later. Therefore, the sacrificing layer 26 is formed of an amorphous silicon film, a polysilicon film or the like which has a high etching selectivity ratio relative to an oxide film, a nitride film, and a metallic film for constituting the substrate-side electrode 20 and the bridge electrode portion 16.
Subsequently, a SiN film is formed on the entire surface of the substrate 18, and it is patterned to form the bridge member 24 that is based on the substrate 18 and is astride and in contact with the sacrificing layer 26, as shown in FIG. 7C.
Next, as shown in FIG. 7D, a membrane-side electrode film consisting of an Al film is formed on the entire surface of the substrate 18 inclusive of the substrate-side electrode facing portion 24a of the bridge member 24, and it is patterned to form the membrane-side electrode 14 on the substrate-side electrode facing portion 24a of the bridge member 24.
Next, the sacrificing layer 26 composed of the amorphous silicon film or the polysilicon film is removed by dry etching using XeF2 gas to complete the MOEMS 12, as shown in FIG. 7E.
Thus, in the fabrication of the MEMS elements, microstructures are formed on a silicon substrate or a glass substrate by application of a surface micro-machining technology based on the fabrication process of a semiconductor integrated circuit for forming a thin film structure on a silicon substrate.
In order to fabricate the above-described microstructures utilizing the elasticity of beams or the like, it is necessary to form the void layers under the beams. Therefore, as described above, the sacrificing layer is preliminarily provided, then other layers for constituting the beam portions are formed on the sacrificing layer, and the sacrificing layer is removed by etching, whereby the void layers are provided and the beam portions are formed.
According to the method of fabricating the MOEMS described above, at the time of removing the sacrificing layer 26 composed of silicon, such as polysilicon and amorphous silicon, XeF2 gas showing a high etching rate ratio between silicon and a material other than silicon is used as an etching gas.
Besides, the sacrificing layer composed of silicon also can be etched away by plasma etching using SF6, NF3 or the like as a reactant gas.
In the etching using XeF2 and in the plasma etching using SF6, NF3 or the like as a reactant gas, the etching selectivity ratio between SiN constituting the membrane and silicon constituting the sacrificing layer is not less than 100, and, in the case of fabricating a single MOEMS, the sacrificing layer composed of silicon can be removed, particularly without damaging the SiN film.
In the case of fabricating such a device as the above-mentioned GLV device 10 comprising a multiplicity of MOEMS 12 disposed densely and in parallel to each other, as shown in FIG. 8A, however, an attempt to etch away the sacrificing layer 26 by feeding the etching gas through the gaps between the membranes 16 results in the etching rate, namely, the progress of etching, is non-uniform between the portions under the membranes 16 and the portions between the membranes 16, as shown in FIG. 8B, and the sacrificing layer 26 cannot be etched uniformly. As shown in FIG. 8B, particularly, the etching of the portions directly under the membranes 16 is retarded. FIGS. 8A and 8B are respectively a plan view showing the layout of the MOEMS 12 and a sectional view of the MOEMS 12 taken along line II—II of FIG. 8A.
An attempt to completely etch away the sacrificing layer 26 in the regions where the etching rate is low leads to the SiN film that is exposed in the regions where the etching rate is high is damaged, resulting in a distribution generated in the film thickness of the membranes.
When a distribution is generated in the film thickness of the membranes, the smoothness of the membranes after completing the void structures is worsened, the light reflectance is lowered, and the light utilization efficiency of the MOEMS is worsened. Further, the movements by electrostatic driving of the membranes are dispersed, and, therefore, predetermined light modulation characteristics are not displayed. Furthermore, the mechanical strength of the membranes is dispersed, and, therefore, the useful life of the MOEMS and, hence, the useful life of the GLV device are shortened.
Accordingly, it has been desired to develop an MEMS element comprising a membrane with such a structure that it is not damaged at the time of etching away a sacrificing layer.